Corresponding author: P. Graham Oliver (
Academic editor: Matthias Glaubrecht
Three dimensional tubular structures of the ctenidium of some thyasirid bivalves are described for the first time. The classification of the thyasirid gill is modified accordingly into five types based on the number of demibranchs, reflection of the filaments and shape of the filaments, either rod, laminar or tubular. The tubular structure is seen in its most modified form in a chemosymbiotic abyssal species from the south-east Atlantic, which is described here as
Chemosymbiotic bivalves typically host their autotrophic
A more complex gill, akin to the tubular form of many lucinids (
Recently I was sent two thyasirids from the Regab pockmark (Gulf of Guinea) to identify and name following their inclusion in the bacterial study of
Using scanning electron microscopy this paper describes the gross structure of these “tubular” gills and compares them with those described by
Undescribed new genus, single specimen, Off Quatsino Sound, Vancouver Island, British Columbia, Canada,
Station J-13 (Benthos 9). SBMNH 83964, 1 specimen.
All specimens had been previously fixed in ethanol or formaldehyde of unknown concentrations. For scanning electron microscopy, gill tissues were excised and cut transversely and longitudinally using a thin razor blade. Tissues were dehydrated in 100% ethanol overnight and critically point dried with liquid CO2 as the intermediate fluid in a Quorum K850 critical point dryer. Dried samples were mounted and gold coated before examination using a Jeol Neoscope SEM.
Both demibranchs are present, the outer extending over approximately half of the inner (Figs
In “
Both demibranchs are present, the outer and inner of approximate equal size (Fig.
With the frontal bands ripped off a regular tubular structure is seen (Fig.
The tubes are lined with densely packed bacteriocytes as is the surface of the laminar zone, the inter lamellar junctions do not bear bacteriocytes. Preservation was not sufficient to acquire detailed images of the
Both demibranchs are present but the outer is about half the depth of the inner (Fig.
The
Cross sections through the median area of five species of
Scanning electron micrographs of the ctenidium of
Scanning electron micrographs of the ctenidium of
Scanning electron micrograph of the ctenidium of
Scanning electron micrograph of the ctenidium of
Classification of gill types found in the
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Type 1R | Type 1 | 2 | + | – | Complete | Rod |
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Type 2aR | Type 2 | 2 | + | –/+ | Complete | Rod | |
Type 2bR | Type 2 | 1 | – | – | Complete | Rod | |
Type 3L | Type 3 | 2 | + | ++ | Complete | Laminar | |
Type 4T/L | 2 | + | ++ | Absent? | Tubular/ laminar |
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Type 5T | 2 | + | +++ | Reduced | Tubular |
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Scanning electron micrograph of the ctenidium of
Scanning electron micrographs of the ctenidium of
Shells and anatomy of granulose thyasirids of the genus
The results presented here reveal further modification of the thyasirid gill towards a complex three-dimensional structure. The gill types described by
The extent of frontal ciliation is noted but the present observations give rise to difficulties in interpretation of the functioning of the tubular/laminar and tubular gills. All Type 3 (L) gills have complete ciliation of frontal and lateral cilia with eulateral cirri as also seen in the
Although there is an indication of frontal ciliation in
Thyasirids typically hold their
This tubular structure increases the surface area of the bacteriocyte zone and creates a more rigid network further facilitating the movement of water from the infra-branchial chamber to the supra-branchial chamber. Similar structures are present within the
The gill structures described here can potentially impact on the systematics of the
Equivalve, Equilateral. Outline subcircular, lunule margin depressed, posterior margin with a single weak sinus. Posterior sulcus shallow but prominent. auricle absent. Escutcheon narrow, shallow. Ligament deeply sunken. Hinge edentulous. External sculpture of commarginal lines and growth stops, overall with microscopic conical, calcareous spines these randomly distributed with some linked by low ridges. Ctenidia of two demibranchs, filaments tubular, “Type 5T”.
Known only from the Regab pockmark off west Africa.
2 specimens (1 broken) in ethanol. Regab Pockmark, Gulf of Guinea, West Africa, RV Meteor GUINECO cruise, M76/3, Dive 215,
Holotype, entire shell from above, NMW.Z.2014.014.00001
14.4 mm long, 13.4 mm high, 4.3 mm half t. Equivalve, Equilateral. Outline subcircular. Anterior dorsal (lunule) margin concave, lunule slightly depressed, ovate, smooth, demarcated by a low ridge. Posterior dorsal (escutcheon) margin almost straight; escutcheon narrow, very shallow, auricle lacking. Posterior ventral margin weakly indented corresponding with a distinct but shallow posterior sulcus. Median area slightly flattened corresponding with slightly straightened ventral margin. Anterior ventral and anterior a continuous broad curve. Ligament prominent but deeply sunken, about half the length of the escutcheon. Hinge edentulous. Sculpture of irregular commarginal lines and growth stops, overall with microscopic conical, calcareous spines emerging through the periostracum, these as isolated individuals, in small clumps or connected by weak sub-concentric ridges. Periostracum relatively thick, brown in colour, shell white. Muscle scars prominent, anterior adductor scar elongate not diverging from the broad pallial line; posterior adductor scar oval.
Mantle edge thick, free except for a junction demarcating the posterior aperture. Foot large, vermiform, toe elongate, heel very small. Lateral pouches large, multi-lobed, lobes cuboid. Ctenidium of two demibranchs, the outer about half the length of the inner; both with fully reflected filaments, these of Type 5T, composed of tubules fused to a median tube with only a small dorsal inter lamellar space.
Named for my son Tomas.
The shell of
There are, therefore, shell and anatomical synapomorphies that separate